1. Field of the Invention
This invention relates generally to a delay device in which an electrical signal is delayed by a specified time interval in passing therethrough, and more particularly to a variable delay device in which the delay time can be varied in response to a control signal applied thereto.
2. Description of the Prior Art
In general, if the length of the signal transmitting path of an electrical delay line is taken as l and the transmitting or propagating velocity of the electric signal through the signal transmitting path of the delay line is taken as v, a delay time .tau..sub.d of the electric signal is expressed as follows: EQU .tau..sub.d =l/v
Various delay lines have been proposed in the art to vary the delay time of electrical signal. Two approaches taken by these known delay lines are to vary the transmitting velocity v and to substantially vary the length l of the signal transmitting path.
If the frequency of an input signal is represented by f.sub.1 and the propagating velocity thereof in the delay line is represented by v.sub.1, the wavelength .lambda..sub.1 of the signal in the delay line is expressed as follows: EQU .lambda..sub.1 =v.sub.1 /f.sub.1
Accordingly, in the case where the delay line varies the propagating velocity v, the wavelength .lambda. is in proportion to the propagating velocity v in the stationary state. However, when this delay line is used in a VTR (Video tape recorder) to correct jitter in a reproduced video signals, the delay time is varied in step like fashion during, the horizontal blanking period H.sub.b. Since the wavelength of the signal applied to the delay line by the propagating velocity v.sub.1 immediately before the variation, is .lambda..sub.1 the output frequency f.sub.z during the transient period immediately after the time when the propagating velocity is changed from v.sub.1 to v.sub.2 to the time when the delay time becomes .tau..sub.d2 is expressed as follows; EQU f.sub.2 =v.sub.2 /.lambda..sub.1 =(v.sub.1 /.lambda..sub.1).multidot.(v.sub.2 /v.sub.1)=f.sub.1 .multidot.(.tau..sub.d1 /.tau..sub.d2)
Thus, this method is defective in that the frequency f.sub.1 of the input signal is different from the frequency f.sub.2 of the delayed output signal during the transient period.
In the known method which substantially varies the length l of the signal transmitting path of the delay line, the above defect does not occur. However, varying the length l requires mechanical operations which prevent high speed changing of the delay time.